UIViewController subclass can't assign instance variable

Question

This is utterly baffling to me, making me think I've missed something essential. But I'm trying to create a view controller container to manage two panes (similar to a SplitViewController but with some crucial differences). I've barely begun constructing the class, so I've only got 3 iVars right now:

@implementation SplitViewController{
    BOOL _showLeftPane;
    UIViewController *_leftController;
    UIViewController *_rightController;
}

No matter what I do, I cannot assign a view controller to the _leftController iVar. Mostly, the _rightController iVar can assign. It get's weirder. If I switch the order of the iVars so _rightController appears before _leftController, I can't assign either one on the first try. However, if I try again (literally write the assignment twice) the _leftController will take. Consistently, when I use the console to request the the value of the first controller in listed in the iVar declarations immediately after assignment, the value I get is (UIViewController *) $5 = 0x00000001 [no Objective-C description available]. WTF?

Out of curiosity (and because I can...I've barely started writing the class) I changed the super from UIViewController to NSObject. When I do that, assignment works. And really, assignment is just about all this class does right now beside create a blank view on loadView.

What am I missing?

Update The rest of the code:

The entire .h file:

#import <UIKit/UIKit.h>

@interface SplitViewController : UIViewController

@property (strong) UIViewController *leftViewController;
@property (strong) UIViewController *rightViewController;
@end

The entire .m file (note, I've tried synthesizing the properties, same behavior except it's harder to debug from the console):

@implementation SplitViewController{
    BOOL _showLeftPane;
    UIViewController *_rightNavController;
    UIViewController *_leftNavController;
}
#pragma mark - Init Methods
- (id)init{
    if (self = [super initWithNibName:nil bundle:nil]){
        _showLeftPane = YES;
    }
    return self;
}
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {

    }
    return self;
}

#pragma mark - Properties
- (void) setLeftViewController:(UIViewController *)leftViewController{
    _leftNavController = leftViewController;
}
- (UIViewController *) leftViewController{
    return _leftNavController;
}
- (void) setRightViewController:(UIViewController *)rightViewController{
    _rightNavController = rightViewController;
}
- (UIViewController *) rightViewController{
    return _rightNavController;
}

Originally, I had all the normal view controller overridden methods (loadView, viewDidLoad, etc...), but I've since commented them out until I could get this problem ironed out. Also, I'm using ARC.

Update 2 I've tried setting up a simple NSObject container class, thinking maybe it had to do with the iVars being of class UIViewController that posed the problem. But the same problem persists. So wrapping the UIViewController into another variable doesn't work. I've also tried changing the iVar type to 'id' but the problem persists.

I'd also like to note that I've cleaned the project and utilized the clang static analyzer to check for logic errors.

Update 3 All assignment in this class are 'off'...often (but not always) assigning the next iVar in the list. I put in a bunch of dummy id iVars and assigned them random objects. I then stepped through each assignment (which I put in the init) and tested the value before and after assignment:

 if (self = [super initWithNibName:nil bundle:nil]) {
        _showLeftPane = YES;
        _dummy1 = [[NSObject alloc] init];
        _dummy2 = [[UIView alloc] init];
        _dummy3 = [[NSNumber alloc] init];
        _dummy4 = [[NSString alloc] init];
        _dummy5 = [[NSValue alloc] init];
        _dummy6 = [[UIColor alloc] init];
        _dummy7 = [[NSCache alloc] init];
        _dummy8 = [[UIButton alloc] init];
        _dummy9 = [[UIFont alloc] init];
        _rightController = [[UIViewController alloc] init];
        _leftController = [[UIViewController alloc] init];
    }

This is what I got (not the values don't change after assignment but those that do have values are often from the previous assignment). :

(lldb) po _showLeftPane
(BOOL) $2 = '\0' <nil>
(lldb) po _showLeftPane
(BOOL) $4 = '\0' <nil>
(lldb) po _dummy1
(id) $6 = 0x00000001 [no Objective-C description available]
(lldb) po _dummy1
(id) $8 = 0x00000001 [no Objective-C description available]
(lldb) po _dummy2
(id) $9 = 0x0a0461e0 <NSObject: 0xa0461e0>
(lldb) po _dummy2
(id) $11 = 0x0a0461e0 <NSObject: 0xa0461e0>
(lldb) po _dummy3
(id) $12 = 0x0866f340 <UIView: 0x866f340; frame = (0 0; 0 0); layer = <CALayer: 0xa05a790>>
(lldb) po _dummy3
(id) $14 = 0x0866f340 <UIView: 0x866f340; frame = (0 0; 0 0); layer = <CALayer: 0xa05a790>>
(lldb) po _dummy4
(id) $15 = 0x00000000 <nil>
(lldb) po _dummy4
(id) $17 = 0x00000000 <nil>
(lldb) po _dummy5
(id) $18 = 0x017c5a7c <object returned empty description>
(lldb) po _dummy5
(id) $20 = 0x017c5a7c <object returned empty description>
(lldb) po _dummy6
(id) $21 = 0x00000000 <nil>
(lldb) po _dummy6
(id) $23 = 0x00000000 <nil>
(lldb) po _dummy7
(id) $24 = 0x0865d300 <UIPlaceholderColor: 0x865d300>
(lldb) po _dummy7
(id) $26 = 0x0865d300 <UIPlaceholderColor: 0x865d300>
(lldb) po _dummy8
(id) $27 = 0x0a230af0 <NSCache: 0xa230af0>
(lldb) po _dummy8
(id) $29 = 0x0a230af0 <NSCache: 0xa230af0>
(lldb) po _dummy9
(id) $30 = 0x08637350 <UIButton: 0x8637350; frame = (0 0; 0 0); opaque = NO; layer = <CALayer: 0x8484920>>
(lldb) po _dummy9
(id) $32 = 0x08637350 <UIButton: 0x8637350; frame = (0 0; 0 0); opaque = NO; layer = <CALayer: 0x8484920>>
(lldb) po _rightController
(UIViewController *) $33 = 0x00000000 <nil>
(lldb) po _rightController
(UIViewController *) $35 = 0x00000000 <nil>
(lldb) po _leftController
(UIViewController *) $36 = 0x0a065180 <UIViewController: 0xa065180>
(lldb) po _leftController
(UIViewController *) $38 = 0x0a065180 <UIViewController: 0xa065180>

So, uh...is this a bug? If so, how in the heck would I file it? I don't even know what's going on....

Update 4 I got it to work, but only on my device. I generally tend to implement major changes on the simulator first (easier/quicker to debug) and then test it for performance on the device. Plus, I prefer to have a working version on my device so I can test it in my spare time. So I installed it on my device and the class works fine. It seems to only be broken in the simulator. Any ideas on why this might be? I really don't want to have to test the rest of my program development on the device alone. That'll slow things down a bit.

Can you give us some more code. Your syntax above seems fine so it must be something else. — Nico Huysamen
– Nico Huysamen, Commented Feb 28, 2012 at 13:31
Well, there's not that much more code to add but I'll add it in a minute. — Aaron Hayman
– Aaron Hayman, Commented Feb 28, 2012 at 13:37

Aaron Hayman · Accepted Answer · 2012-04-02 13:22:09Z

9

Ok, I figured out the problem. I'm using Xcode 4.3, which is supposed to allow you to use the lldb debugger with iOS (or so Apple says). My problem went away when I changed the debugger to GDB. Though now I have a useful debugger, I've realized that the assignments were actually working correctly. The debugger was just spitting back the incorrect data. Gotta love it.

My advice: Don't use lldb on iOS. Even though Apple says the lldb debugger can be used with iOS, it's still not stable enough to rely on. So yeah, there went 6 hours of my life....

I've filed a bug with Apple: 10945570

UPDATE

Apple seems to have fixed this in Xcode 4.3.1. At least, so far LLDB is working well for me.

UPDATE 2

No, nevermind, LLDB is still messed up in the simulator.

UPDATE 3

So far as I can tell, this issue is now resolved in 4.3.2. I've been using the LLDB debugger now for about a week and I haven't had any problems with it giving me garbage values.

edited Apr 2, 2012 at 13:22

answered Feb 28, 2012 at 16:02

Aaron Hayman

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1 Comment

blahdiblah Over a year ago

Good on ya for providing all the follow-up.

Community · Accepted Answer · 2017-05-23 12:17:20Z

2

This code does not seem correct:

@interface SplitViewController : UIViewController

@property (strong) UIViewController *leftViewController;
@property (strong) UIViewController *rightViewController;
@end

....

@implementation SplitViewController{
  BOOL _showLeftPane;
  UIViewController *_rightNavController;
  UIViewController *_leftNavController;
}

Is this what you really have? Because I would have expected something like this in the .h:

@interface SplitViewController : UIViewController
@property BOOL showLeftPane;
@property (strong) UIViewController* leftViewController;
@property (strong) UIViewController* rightViewController;
@end

And in the .m

@implementation SplitViewController
@synthesize showLeftPane;
@synthesize leftViewController;
@synthesize rightViewController;
...
@end

Or if you wanted to use explicit backing vars

@interface SplitViewController : UIViewController
{
    BOOL _showLeftPane;
    UIViewController* _leftViewController;
    UIViewController* _rightViewController;
}

@property BOOL showLeftPane;
@property (strong) UIViewController* leftViewController;
@property (strong) UIViewController* rightViewController;
@end

....

@implementation SplitViewController
@synthesize showLeftPane = _showLeftPane;
@synthesize leftViewController = _leftViewController;
@synthesize rightViewController = _rightViewController;
...
@end

From the posted code it looks like you are mixing up concepts between the @interface and the @implementation

As per the your comment, if you want private vars then the .m file should be something like:

@interface SplitViewController()
{
  BOOL showLeftPane;
  UIViewController* leftViewController;
  UIViewController* rightViewController;
}
@end

....

@implementation SplitViewController
...
@end

Actually I'll probably take that back. Not sure it works for vars .. but it does for methods.

YOu can make vars private through the use of @private in the interface

edit

My mistake. The original code is correct, in that you can put iVars in the implementation file like that since XCode 4.2. As per this SO question instance-variables-declared-in-objc-implementation-file you need 4.2+ and LLVM selected.

So now I sound like I don't know what I am talking about!

edited May 23, 2017 at 12:17

CommunityBot

11 silver badge

answered Feb 28, 2012 at 14:41

Peter M

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8 Comments

Aaron Hayman Over a year ago

Well, as I mentioned in my question, I did try synthesizing the properties. But regardless, I'm not using the properties as I troubleshoot. I'm directly assigning them in the init file. Also, there is no need to have the iVars in the .h file. I prefer them in the .m file anyway since they're private by nature. At any rate, I tried putting them in the .h file. Same behavior.

Aaron Hayman Over a year ago

Ok, for kicks I tried it. Still doesn't work. But I can't see why you would want to put the iVars in a category. They work just fine in the implementation. In fact, every one of my 100+ classes in this project have their iVars in the implementation. I remember there was a time you couldn't do that, but objective-c has allowed iVar declarations in the @implementation for a little while now.

Peter M Over a year ago

OK .. my mistake on the location of the iVars. I didn't know you could do that. But to check, as per this SO question stackoverflow.com/questions/6785765/… you need XCode 4.2+ and LLVM selected. Is that true?

Aaron Hayman Over a year ago

Well, by 'private' I mean 'not visible in the .h' file. @private just prevents subclasses from accessing the iVar directly. But anybody can technically access any class iVars by using the c++ construct: ->, if they know the iVar name. Leaving the iVar declarations out of the .h file is just good form IMHO.

Peter M Over a year ago

@AaronHayman Now that I know you can do that I will start doing it. I totally agree that you should only expose what is needed. As a result I am sorry for going off blindly.

|

Oscar Gomez · Accepted Answer · 2012-02-28 13:29:02Z

0

Maybe for some reason the #import <UIKit/UIKit.h> is not on your precompiled headers, try adding it there, or simply adding the import on top of your file.

answered Feb 28, 2012 at 13:29

Oscar Gomez

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1 Comment

Aaron Hayman Over a year ago

Nice thought, but no, it's there.

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